This invention relates to multilayer ceramic circuit boards mounted on a patterned conductive substrate support and method of making. More particularly, this invention relates to mounting modules of high power amplifiers and oscillators onto the patterned substrate support to remove excess thermal energy from these devices.
U.S. Pat. No. 5,847,935 to Thaler et al describes an electronic circuit chip package which includes a base plate of an electrically and/or thermally conductive material, such as a metal plate, having a body of an insulator adhered to the base plate. This insulator body may comprise a plurality of glass-ceramic layers having printed circuitry thereon. Conductive vias through the ceramic layers connect the circuits electrically to each other. At least one opening is formed in the insulator body through to the support substrate, the opening having a size so as to accommodate at least one electronic device or module. The electronic device is mounted directly onto the conductive base plate. Wires are used to connect terminals in the electronic device to terminal strips in the circuitry.
This technology is known as low temperature co-fired ceramic circuit boards with metal support substrates, or LTCC-M technology. This technology permits the inclusion of active and passive devices between the glass-ceramic layers, as well as the inclusion of transmission lines, capacitors, resistors and the like.
When mounting high power amplifier and oscillator electronic devices to a substrate, a common problem is that of removing excess thermal energy from the devices, so that the active devices and circuit components are maintained at operating temperatures consistent with reliable performance.
The metal substrate support of the above-described LTCC-M technology can serve also as a heat spreader, or heat sink, which is advantageous in that modules can be directly mounted to the metal substrate such that they have an improved heat energy path. Thus by mounting high power components, such as amplifiers and oscillators, in an opening through the glass-ceramic layers directly onto a metal substrate support, minimal heat resistance to the heat sink can be achieved. The failure rate (MTTF) of power amplifiers and oscillators, as well as other high power devices, can thus be reduced markedly.
However, it would be desirable to be able to surface mount the above LTCC-M modules to a motherboard for increased integration. In order to do that, the signal-power leads can be desirably soldered directly to appropriate contacts on the motherboard. Such a configuration requires that the power and signal leads of the module are brought through the heat sink. Such a configuration would improve reliability and reduce manufacturing costs. However, forming openings through the metal support substrate in a completed LTCC-M device is expensive and presents various problems of manufacturability. Thus heat sinked modules for high power devices that can be readily made at low cost would be highly desirable.
We have found that by patterning the metal support in an LTCC-M module so as to be able to provide openings through the metal support prior to mounting one or more high power modules thereon, and applying solder pads to both sides of the support, the power and the signal leads of devices thereon, and heat sinking of the module, can all be achieved. Through vias in the ceramic layers of the LTCC-M module are directly connected to the top side of the metal support substrate, which also acts as a heat sink for a high power module.
Another aspect of the invention includes a dielectric paste that is screen printed onto the bottom side of the LTCC-M module so that the dielectric paste is aligned with the openings in the metal substrate support. This dielectric paste prevents shrinkage in the x and y dimensions of the green tapes mounted in those areas that overlie the openings in the metal substrate. After laminating the prepared module to the prepared substrate, and firing, the dielectric, now a powder, is readily removed.
This design further permits high integration because the above heat sink and module and other circuitry can be directly soldered from the bottom side of the substrate support to solder pads on a motherboard, which can include additional devices.